Computers are often connected together as part of a Local Area Network (LAN). The LAN permits computers to share data and programs with one another. Many typical LANs are based upon physical connections between individual computers and a server, for example. The connections may be twisted pair conductors, coaxial cables, or optical fibers, for example.
There is also another class of LAN based upon wireless communication to the individual computers. A wireless LAN is not restricted to having physical connections to the individual computers. Accordingly, original installation may be simplified. Additionally, one or more of the computers may be used in a mobile fashion. In other words, the user may use a laptop computer and move from place to place while still being connected via the wireless LAN.
Various standards have been created to define operating protocols for wireless LANs, such as the IEEE 802.11 and Bluetooth standards. The IEEE 802.11 standard, for example, defines the protocol for several types of networks including ad-hoc and client/server networks. An ad-hoc network is a network where communications are established between multiple stations in a given coverage area without the use of an access point or server. The standard provides methods for arbitrating requests to use the medium to ensure that throughput is maximized for all of the users in the base service set. Another widely used wireless LAN format is Bluetooth.
The client/server network uses an access point that controls the allocation of transmit time for all stations and allows mobile stations to roam from one access point to another. The access point is used to handle traffic from the mobile radio to the wired or wireless backbone of the client/server network. This arrangement allows for point coordination of all of the stations in the basic service area and ensures proper handling of the data traffic. The access points route data between each station and other wired/wireless stations, or to and from the network server (i.e., a base station). Of courser two or more LANs may be interconnected using wireless LAN devices at respective access points. This may be considered a network bridge application.
One of the challenges of wireless LAN implementation is designing suitable antennas that can provide desired performance characteristics, yet are relatively small in size to fit within mobile devices. For example, with wireless LAN devices such as laptop computers, it is desirable to keep the overall size of the laptop as small as possible. Furthermore, internal antennas are generally preferred over external antennas, as externally mounted antennas take up more space and may be damaged while traveling, etc.
One example of a wireless LAN antenna that is implemented on a PMCIA card to be inserted in a PMCIA slot of a laptop computer is disclosed in U.S. Pat. No. 6,031,503 to Preiss, II et al. The antenna assembly includes two folded, U-shaped antennas, which may be dipoles or slot radiators, that are disposed orthogonally to one another to provide polarization diversity. Polarization diversity means that signals are transmitted and received on two different polarizations to increase the likelihood that the signal is received. Signals are carried to and from the antenna by microstrip feed lines. The microstrip lines are placed off center along each antenna slot to establish an acceptable impedance match for the antenna, and the feed lines are coupled to the communications card by coaxial cables.
There is an increasing trend toward using other portable, handheld communications devices in wireless LANs which are even smaller than laptops, such as personal digital assistants (PDAs), for example. Accordingly, with even more restrictive space constraints for such handheld devices, there is a need for antennas which are appropriately sized for such applications yet still provide desired performance characteristics.